1. Field of the Invention
The present invention relates to a lens unit that forms an image of image information of a document on an image sensor, and an image reading device including the lens unit, and an image forming apparatus including the image reading device.
2. Description of the Related Art
An image forming apparatus such as a copying machine, a printer, a facsimile machine, and a multifunction peripheral is provided with a lens unit that forms an image of image information of a document on an image sensor.
Conventionally known has been an image reading lens (lens unit) which is provided with a lens group including a plurality of lenses and a single lens arranged at a distance from the lens group as disclosed in Japanese Laid-open Patent Publication No. 2013-54295, for example. This lens unit has an advantage of preventing a degradation of lens characteristics such as a resolution while making a conjugation length short and diameters of lenses small because of the distance between the lens group and the single lens.
Besides, known has been another lens unit provided with a tube part (tube lens) including at least one lens, a single lens arranged to be in contact with the tube part, and an elastic member that depresses the single lens to the tube lens as disclosed in Japanese Patent No. 4994753, for example.
However, Japanese Laid-open Patent Publication No. 2013-54295 does not disclose anything about how to support the lens group and the single lens.
There is a limitation in trying to make a conjugation length short and an angle of view wide in the lens unit disclosed in Japanese Patent No. 4994753 since the tube lens and the single lens are in contact and a distance between the tube lens and the single lens cannot be extended.
A method of making the tube lens and the single lens supported by a board can be considered to extend the distance between the tube lens and the single lens. However, the board as well as the lenses would have a thermal deformation caused by changes in temperature and the distance between the tube lens and the single lens would thereby be extended and contracted in the method, leading to a concern about a degradation of lens characteristics such as a resolution.
Therefore, there is a need for a lens unit capable of suppressing the extension and contraction, caused by changes in temperature, of the distance between the tube lens and the single lens.